No abstract

The Azeri-Chirag-Gunashli (ACG) field, a tightly folded anticline structure, located offshore Azerbaijan in the south Caspian basin, is one of the most tectonically active regions of the world. Understanding the stress state across the ACG structure is a key to successful development of the field by optimizing well placement during drilling, completion, and depletion/injection phases. This article summarizes the results of studies undertaken on the state of the stress in ACG field. It encompasses the field-wide overview of stresses from a structural standpoint, the compilation of drilling and completion events, for instance, induced fracture during lost circulation events, and Formation Pressure Integrity Tests (FPIT) as well as analysis of wellbore breakout from variety of sources including borehole image data and caliper logs that were used to infer the magnitude and orientation of far-field stresses. Key outcomes of this study are the stress ratio distribution maps and stress orientation maps across the structure, based on the magnitude and orientations of stresses that were inferred from drilling events and wellbore breakouts. Results of this analysis show that magnitude and orientation of stresses vary across the structure both laterally and vertically. Minimum principal stress (Shmin) tends to increase from the crest toward the flanks. The maximum principal stress (SHmax) orientation is found to be predominantly sub-perpendicular to the strike of the anticline structure (60°-80° N), influenced by the reginal tectonic stresses. Moreover, stress rotation from sub-perpendicular to sub-parallel to the anticline is observed over some parts of the central and crestal areas, indicating that stresses are less compressional at the center and crest of the Azeri anticline. Local variability is possibly due to proximity to geological features such as mud volcanos, faults, and high deformation areas. The relative magnitude of stresses found in ACG, suggests a predominantly strike-slip faulting regime (where SHmax is the greatest of the three principal stresses) particularly, at the flanks and noses of the structure.